1. Field of the Invention
The present invention relates to an electron source, an image forming apparatus, and a manufacture method for an electron source.
2. Related Background Art
Many types of apparatus are known in which a number of electron emitting devices and wirings connected to the devices are disposed on a substrate to form a plane type electron source and an electron beam is emitted from a desired electron emitting device to display an image. For example, the publication of U.S. Pat. No. 5,942,849 (Neil Alexander Cade) discloses an apparatus in which electron emission from a field emitter chip is controlled by two grid electrodes (wirings) crossing each other at a right angle. In this apparatus, an electron emitting device is disposed at a cross point between the wirings. Another structure is also known in which an electron emitting device is disposed near at the wiring cross point in an area of the substrate where the wiring is not formed. The present applicant has already proposed an apparatus having such a structure. For example, this apparatus is disclosed in the publication of U.S. Pat. No. 5,654,607.
Electron emitting devices are roughly classified into thermal electron emitting devices and cold cathode electron emitting devices. As cold cathode electron emitting devices, a field emission type (hereinafter called an xe2x80x9cFE typexe2x80x9d), a metal/insulator/metal type (hereinafter called an xe2x80x9cMIM typexe2x80x9d), surface conduction electron emitting devices and the like are known.
Examples of the FE type are those disclosed in xe2x80x9cField emissionxe2x80x9d by W. P. Dyke and W. W. Dolan, Advance in Electron Physics, 8, 89 (1956), xe2x80x9cPhysical Properties of thin-film field emission cathodes with molybdenium conesxe2x80x9d by C. A. Spindt, J. Appl. Phys., 47, 5284 (1976) and the like.
Examples of the MIM type are those disclosed in xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d by C. A. Mead, J. Appl. Phys., 32, 646 (1961) and the like.
Examples of the surface conduction type electron emitting device are those disclosed by M. I. Elinson in Recio Electron Phys., 10, 1290 (1965) and the like.
Surface conduction electron emitting devices utilize the phenomenon that electron emission occurs when current is flowed through a thin film having a small area formed on an insulating substrate in parallel to the film plane.
Reports on surface conduction electron emitting devices show those using SnO2 thin films by Elinson or the like, those using Au thin films (xe2x80x9cThin Solid Filmsxe2x80x9d by G. Dittmer, 9, 317 (1972)), those using In2O3/SnO2 thin films (by M. Hartwell and C. G. Fonstad in xe2x80x9cIEEE Trans. ED Conf., 519 (1975)), those using carbon thin films (xe2x80x9cVacuumxe2x80x9d by Hisashi ARAKI, et. al., Vol. 26, No. 1, p. 22 (1983)), and the like.
As a typical example of these surface conduction electron emitting devices, the device structure by M. Hartwell is schematically shown in FIG. 19. On a substrate 401, an electroconductive film 404 having an H-character shaped pattern and made of a sputtered metal oxide thin film is formed. An electron emitting region 405 shown by hatching in FIG. 19 is formed by an operation called an energization forming operation to be described later. A device electrode distance L shown in FIG. 19 is set to 0.5 to 1 mm and Wxe2x80x2 is set to 0.1 mm.
Generally, prior to electron emission of the surface conduction electron forming device, the electroconductive film 404 is subjected to the operation called an energization forming operation to form the electron emitting region 405. With the energization forming operation, a voltage is applied between opposite ends of the electroconductive film 404 to locally destruct, deform, or decompose the electroconductive film 404 and change the structure thereof to thereby form the electron emitting region 405 having an electrically high resistance. Fissures 1 are partially formed in the electron emitting region 405 of the electroconductive film 404. Electrons are emitted nearly from these fissures.
Since the surface conduction electron emitting device has a simple structure, it has the advantage that a number of devices can be arranged in a large area. Various applications utilizing such characteristics have been studied. For example, applications to a charged beam source, an image forming apparatus such as a display apparatus and the like are known.
One example of an electron source having a number of surface conduction electron emitting devices is an electron source (e.g., publications of JP-A-64-031332, JP-A-1-283749, JP-A-2-257552 and the like) in which a number of rows are disposed (in a lattice type) and both ends (two device electrodes) of each of surface conduction electron emitting devices disposed in parallel are connected by wirings (common wires).
Surface conduction electron emitting devices can be used for a flat apparatus, particularly a display apparatus similar to liquid display apparatus which is of a self light emission type requiring no back light. Such a display apparatus is disclosed in the publication of U.S. Pat. No. 5,066,883 in which an electron source having a number of surface conduction electron emitting devices is combined with a fluorescent member which emits visual light when an electron beam is applied from the electron source.
The present applicant has also disclosed an example of an image displaying apparatus in the publication of JP-A-6-342636 in which an electron source with surface conduction electron emitting devices having a wiring pattern whose outline structure is schematically shown in FIG. 20. In FIG. 20, a plurality of surface conduction electron emitting devices are connected in a matrix shape by upper wirings 73 and lower wirings 72.
FIG. 21A is a plan view showing the structure of a surface conduction electron emitting device, and FIG. 21B is a cross sectional view of the surface conduction electron emitting device taken along line 21Bxe2x80x9421B shown in FIG. 21A. The surface conduction electron emitting device has: a pair of electrodes 202 and 203 formed on an insulating substrate 201; an electroconductive thin film 204 made of fine particles and electrically connected to the electrodes 202 and 203; and an electron emitting region 205 formed partially in the electroconductive thin film 204 for emitting electrons. In this surface conduction electron emitting device, a distance between the pair of electrodes 202 and 203 is set to several ten thousand nm to several hundred xcexcm, and the length of the device electrode is set to several xcexcm to several hundred xcexcm by taking into consideration the resistance of the device electrode and the electron emission characteristics. The thickness of the device electrode is set in a range from several thousand nm to several xcexcm in order to retain the electrical connection to the electroconductive film 204. For example, the electrodes 202 and 203 are formed by photolithography techniques. The thickness of the electroconductive film 204 is set properly by taking into consideration the step coverage to the electrodes 202 and 203, the resistance between the device electrodes, the energization forming conditions and the like. The thickness of the electroconductive film 204 is preferably set in a range from several ten nm to several ten thousand nm, or more preferably in a range from 100 nm to 5000 nm. The sheet resistance Rs of the electroconductive film is preferably set to 102 to 107 xcexa9/xe2x96xa1. Rs is given by R=Rs(l/w) where R is a resistance of a thin film having a thickness t, a width w and a length l as measured in the longitudinal direction. If the thickness t and a resistivity xcfx81 are Constance, then Rs=xcfx81/t.
FIG. 22 is a schematic diagram showing an example of the structure of an image display apparatus using an electron source with a plurality of surface conduction electron forming devices wired in a matrix format, as disclosed in the above-cited publication of JP-A-6-342636. A rear plate 81, an outer frame 82 and a face plate 86 are adhered together at their connection points and sealed by unrepresented adhesive such as low melting point glass frit to thereby constitute an envelope (hermetically sealed container) 88 which retains vacuum of the inside of the image display apparatus. A substrate 71 is fixed to the rear plate 81. On this substrate 71, mxc3x97n surface conduction electron emitting devices are arranged (where m and n are positive integers of 2 or larger which are properly determined in accordance with an objective number of display pixels). As shown in FIG. 22, the surface conduction electron emitting devices 74 are wired by m row-directional wires 72 and n column-directional wires 73. For example, these wires 72 and 73 are formed by photolithography techniques. The structure constituted by the substrate 71, a plurality of electron emitting devices 74 such as surface conduction electron emitting devices, row-directional wires 72 and column-directional wires 73 is called a multi electron beam source. Unrepresented interlayer insulating films are formed at least at the cross points between the row-directional and column-directional wires 72 and 73 to retain electrical insulation between both the wires 72 and 73.
A fluorescent film 84 made of fluorescent material is formed on the bottom surface of the face plate 86, the film 84 being divisionally coated with three primary color fluorescent materials (not shown) of red (R), green (G) and blue (B). A black body (not shown) is disposed between the fluorescent materials of the respective colors of the fluorescent film 84. A metal back 85 made of Al or the like is formed on the fluorescent film 84 on the side of the rear plate 81.
Dx1 to Dxm and Dy1 to Dyn are electrical connection terminals of a hermetic seal structure for electrically connecting the image display apparatus and an unrepresented electric circuit. Dx1 to Dxm electrically connect the multi electron beam source to the column-directional wires. Similarly, Dy1 to Dyn electrically connect the multi electron beam source to the row-directional wires.
The inside of the envelope (hermetically sealed container) is maintained vacuum at 1.33xc3x9710xe2x88x924 Pa or lower. Therefore, as the display screen of the image display apparatus is made larger, the means for preventing the rear plate 81 and face plate 86 from being deformed or destructed by a pressure difference between the inside and outside of the envelope (hermetically sealed container) is much more required. It is therefore necessary to dispose support members (not shown) called spacers or ribs between the face plate 86 and rear plate 81 in order to be resistance against the atmospheric pressure.
The distance between the substrate 71 formed with the electron emitting devices and the face plate 86 formed with the fluorescent film is usually set to several hundred xcexcm to several mm, and the inside of the envelope (hermetically sealed container) is maintained high vacuum. With the image display apparatus described above, electrons are emitted from each surface conduction electron emitting device by applying a voltage thereto via the external terminals Dx1 to Dxm and Dy1 to Dyn and via the row- and column-directional wires 72 and 73.
At the same time when the voltage is applied, a high voltage of several hundred V to several kV is applied to the metal back 85 via the external terminal. Electrons emitted from the surface conduction electron emitting device is therefore accelerated and collided with each color fluorescent member formed on the inner surface of the face plate 86. The fluorescent member is therefore excited so that light is emitted and an image is displayed.
In order to manufacture the image display apparatus described above, it is necessary to dispose a number of electron emitting devices and row- and column-directional wires.
As techniques used for forming a number of electron emitting devices and row- and column-directional wires, photolithography techniques, etching techniques and the like are used.
However, if an image display apparatus having a large screen, e.g., several ten inches and using surface conduction electron emitting devices is formed by using photolithography techniques and etching techniques, it is necessary to use large scale manufacture facilities such as a vacuum deposition system, a spin coater, an exposure system, an etching system and the like suitable for a large substrate having a diagonal distance of several ten inches. This poses the problems of control hardness of manufacture processes and high cost.
Printing techniques are known which can form a number of electron emitting devices and row- and column-directional wires of an image display apparatus of a large screen area, as disclosed in the publication of JP-A-9-293469 by the present applicant.
The present applicant disclosed the techniques of forming a number of row- and column-directional wires by using screen printing techniques in JP-A-8-34110.
Screen printing is suitable for forming a thick wiring layer through which large current can be flowed to some degree. By using as a mask an impression formed with openings having a predetermined pattern, print paste mixed with, e.g., metal particles is transferred through the openings to a substrate to be printed, and thereafter the substrate is baked to form electroconductive wires having a desired pattern.
Screen printing will be described with reference to FIG. 23 which is a perspective view of a screen mesh 42 of a screen printing machine and a substrate 100 and with reference to FIG. 24 which is a cross sectional view of the screen mesh 42 and substrate 100 shown in FIG. 23.
In order to make it easy to explain the printing state, an impression frame 41 and screen mesh 42 are shown partially broken in FIG. 23.
First, the outline of screen printing will be described.
As shown in FIG. 23, the screen mesh 42 is suspended by an impression frame 41 by a properly set tensile force. The screen mesh 42 is made of a mesh plate made of stainless steel or the like and a resin film formed thereon. An impression pattern 45 is cut through the resin film to eject print paste 47 via this cut pattern. The print paste 47 to be printed on a substrate 100 is developed on the screen mesh 42 with the impression pattern 45. As the screen mesh 42 is scanned while it is pushed by a squeegee 43, a print pattern 46 is printed on the substrate 100.
Next, the process of screen printing will be described.
First, the surface of the screen mesh 42 suspended by the impression frame 41 and the substrate 100 are set so as to have a predetermined gap 48. Next, the squeegee 43 is lowered until the screen mesh 42 becomes in contact with the substrate 100 at a pushing point 44. Next, the print paste 47 is developed in front of the squeegee 43. While the squeegee 43 is maintained lowered so as to always make the screen mesh 42 in contact with the substrate 100, the squeegee 43 is scanned in a direction indicated by an arrow E in FIG. 23 to scrape off the print paste 47. At this time, by the pressure supplied from the squeegee 43, the print paste 47 is transferred to the substrate 100 via the impression pattern 45. At the same time, the screen mesh 42 is separated from the substrate 100 by a recovery force generated by the vertical components of the tensile force 44 at the pushing point of the screen mesh. The print paste 47 is therefore separated from the screen mesh 42 and a desired print pattern 46 shown in FIG. 23 is formed on the substrate 100.
In the electron source having wiring groups (hereinafter called xe2x80x9crow-directional wiresxe2x80x9d and xe2x80x9ccolumn-directional wiresxe2x80x9d) crossing at a right angle each having a plurality of wires and a plurality of electron emitting devices connected to the wires, although the substrate surface excepting the peripheral surface is finely divided by the row- and column-directional wires, at least ones of the row- and column-directional wires are not disposed on the peripheral surface. For example, in an electron source shown in FIG. 25 which has three row-directional wires and three column-directional wires for supplying a power to nine electron emitting devices disposed in a 3xc3x973 matrix shape on a substrate 402, an exposed area other than the wires is relatively broad and charged regions having a large charge amount in the surface area are likely to be formed as shown. In the case such as shown in FIG. 25 wherein the electron emitting device is formed not at the cross point of the wires but in the surface layer of the substrate 402, the row-directional wire is not formed between the peripheral surface of the substrate 402 and ones of outer side devices 401 in the row direction. There is, therefore, a danger that these outer side devices 401 are greatly influenced by the charged areas having an increased charge amount on the surface of the substrate 402. The same danger occurs for the outer side devices in the column direction.
The problem of an increased charge amount of the substrate surface near the device may be associated with the following disadvantages.
(1) The charge amount of the substrate surface near the electron emitting devices belonging to the row without the outer side row-directional wire and to the column without the outer side column-directional wire is larger than that of other electron emitting devices. Therefore, the distribution of an electric field near each device becomes different. The electron emission characteristics are therefore different therebetween and uniformity of the electron emission characteristics is degraded.
(2) Since the distribution of the electric field near each device becomes different, the trajectory of an emitted electron also becomes different. Therefore, the effective uniformity of the electron source is further degraded.
(3) The charge amount of the substrate surface also changes with the device drive conditions (drive voltage, drive pulse width and the like). This becomes more conspicuous to the electron emitting device without the outer side row or column wire than other devices. A change in the characteristics caused by (1) and (2) varies greatly with time, and the fluctuation of the characteristics is especially great in the device without the outer side row or column wire than other devices.
(4) A large charge amount may cause discharge between the charged area of the substrate surface and the device, wires and the like. This discharge may damage the electron emitting device, so that the electron emission amount may reduce or the device may be destroyed.
The following problem may occur when a plurality of parallel wires are formed by the screen printing method described above. As described earlier, screen printing is executed by pushing the mesh screen against the substrate. When the screen mesh separates from the substrate after print paste is transferred to the substrate via the pattern of the screen mesh, two forces are exerted on the pushing point of the screen mesh. One force results from the tensile force of the screen mesh to separate the screen mesh from the substrate, and the other force is applied from the print paste transferred to the substrate to adhere the screen mesh to the substrate. While one wiring pattern is printed, other wiring patterns on both sides of the one wiring pattern are also formed. The screen mesh separates from the substrate while being influenced by the adhesion force of each wiring pattern. Therefore, of a number of parallel wiring patterns, the pattern near the central area and the pattern in the outer side area receive different adhesion forces. The wiring pattern, particularly the outermost wiring pattern does not receive the adhesion force at the area outer than this pattern, so that the transfer of print paste becomes likely to be irregular. The shape defect of the print pattern may occur, possibly resulting in contact defects between wires and device electrodes, wire resistance distribution, high resistance in the peripheral area, wire disconnection, and the like.
It is an object of the invention to provide an electron source and an image forming apparatus having good characteristics, and a manufacture method for such an electron source. Specific embodiments of the invention can solve at least one of the problems specifically described above.
The present invention provides an electron source having a plurality of first wires and a plurality of electron emitting devices respectively formed on a substrate, the first wires having a longitudinal direction generally along a first direction and the electron emitting devices being connected to each of the first wires, comprising: at least one first conductor formed between first outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate, and near the first outer electron emitting devices, the first wire not being formed between the outer periphery and the first outer electron emitting devices, the first conductor having a side on a side of the first outer electron emitting devices, the side extending generally along the first direction, wherein the conductor is not connected to electron emitting devices connected directory in a wire to which at least some of the plurality of electron emitting devices are connected.
With this structure, the first conductor can suppress electric charges and/or can mitigate the adverse effects of electric charges.
The invention is particularly effective if each electron emitting device is formed at a position different from a position where each first wire is formed.
The electron source may further comprise at least one second wire formed on the substrate, the second wire having a longitudinal direction generally along a second direction crossing the first direction, each electron emitting device is connected to one of the first wires and the second wire. The invention is effectively applicable to the structure wherein the electron emitting device is formed in an area different from the areas where the first and second wires are formed.
If the second wire is used and there is an electron emitting device connected to the first conductor and second wire, undesired charge transfer may occur by a potential difference between the first conductor and second wire. It is therefore preferable not to connect the electron emitting devices connected to the second wire, to the first conductor.
The electron source may further comprise a plurality of second wires formed on the substrate, the second wires having a longitudinal direction generally along a second direction crossing the first direction, wherein each electron emitting device is formed at a cross point between each of the first wires and each of the second wires and connected to the first wire and the second wire crossing at the cross point.
The electron source may further comprise at least one second conductor formed between second outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate at least on one side of the substrate, and near the second outer electron emitting devices, the second wire not being formed between the outer periphery and the second outer electron emitting devices, the second conductor having a side on a side of the second outer electron emitting devices, the side extending generally along the second direction. It is also preferable not to connect the electron emitting devices to be driven to the second conductor. It is preferable that the electron emitting devices connected to the second wire are not connected to the first conductor and the electron emitting devices connected to the first wire are not connected to the second conductor.
It is preferable that the second conductor is electrically connected at least any of the wires, and more preferably, the second conductor is electrically connected to the second wire.
The electron source may further comprise: a plurality of second wires formed on the substrate, the second wires having a longitudinal direction generally along a second direction crossing the first direction, wherein each electron emitting device is formed at a cross point between each of the first wires and each of the second wires and connected to the first wire and the second wire crossing at the cross point; and at least one second conductor formed between second outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate at least on one side of the substrate, and near the second outer electron emitting devices, the second wire not being formed between the outer periphery and the second outer electron emitting devices, the second conductor having a side on a side of the second outer electron emitting devices, the side extending generally along the second direction, wherein the second conductor is electrically connected to the second wire excepting the second wire nearest to the second conductor.
Electric charges can effectively suppressed if a distance between the second conductor and the second wire nearest to the second conductor is set to a twofold of or smaller than a distance between the adjacent second wires. More preferably, a distance between the second conductor and the second wire nearest to the second conductor is set generally equal to a distance between the adjacent second wires. The generally equal distance means that a difference between distances is 20% or smaller than the distances. The distance means a gap between the sides of adjacent wires in the longitudinal direction. If the distance is not constant, the average value is used.
It is preferable that a plurality of second conductors are formed adjacent to each other at a distance shorter than a distance of the adjacent second wires.
It is preferable that a resistance value of the second conductor is set to a tenfold of or smaller than a resistance value of the second wire. More preferably, the resistance value of the second conductor is set generally equal to that of the second wire.
The invention is particularly effective if the second wire is applied with a signal for driving the electron emitting device.
It is preferable that the first conductor is electrically connected to the first wire. This structure is particularly effective. It is preferable that the first conductor is electrically connected to the first wire excepting the second wire nearest to the first conductor.
It is also preferable that a plurality of first conductors are formed adjacent to each other at a distance shorter than a distance between the adjacent first wires.
It is preferable that a distance between the first conductor and the first wire nearest to the first conductor is set to a twofold of or smaller than a distance between the adjacent first wires. More preferably, a distance between the first conductor and the first wire nearest to the first conductor is set generally equal to a distance between the adjacent first wires.
It is preferable that a resistance value of the first conductor is set to a tenfold of or smaller than a resistance value of the first wire. More preferably, a resistance value of the first conductor is set generally equal to that of the first wire.
It is preferable that the first wire is applied with a signal for driving the electron emitting device. For example, a selection signal is sequentially applied to the plurality of first wires to scan the electron emitting devices. A modulation signal may be applied to the first wires. More specifically, a selection signal is sequentially applied to the plurality of first wires to scan the electron emitting devices and a modulating signal is applied to the second wires to properly scan the electron source.
The present invention covers following structures of the electron sources, which can be used in combinations of the above described structures.
The invention also provides an electron source having a plurality of first wires, a plurality of second wires, and a plurality of electron emitting devices respectively formed on a substrate, the first wires having a longitudinal direction generally along a first direction, the second wires having a longitudinal direction generally along a second direction crossing the first direction, and the electron emitting device being connected each of the first wires and each of the second wires at a cross point therebetween, comprising: at least one first conductor formed between first outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate, and near the first outer electron emitting devices, the first wire not being formed between the outer periphery and the first outer electron emitting devices, the first conductor having a side on a side of the first outer electron emitting devices, the side extending generally along the first direction; and at least one second conductor formed between second outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate, and near the second outer electron emitting devices, the second wire not being formed between the outer periphery and the second outer electron emitting devices, the second conductor having a side on a side of the second outer electron emitting devices, the side extending generally along the second direction.
The scope of the present invention covers also following structures. The following structures belongs to the scope of the above described structure, but can be desirably used in combination of the above described structure.
The invention also provides an electron source having a plurality of first wires and a plurality of electron emitting devices respectively formed on a substrate, the first wires having a longitudinal direction generally along a first direction and the electron emitting devices being connected to each of the first wires, comprising: a plurality of first conductors formed between outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate, and near the outer electron emitting devices, the first wires not being formed between the outer periphery and the outer electron emitting devices, the first conductors having a side on a side of the first outer electron emitting devices, the side extending generally along the first direction.
The invention also provides an electron source having a plurality of first wires and a plurality of electron emitting devices respectively formed on a substrate, the first wires having a longitudinal direction generally along a first direction and the electron emitting devices being connected to each of the first wires, comprising: at least one first conductor formed between outer electron emitting devices among the plurality of electron emitting devices and an outer periphery of the substrate, and near the outer electron emitting devices, the first wire not being formed between the outer periphery and the outer electron emitting devices, the first conductor having a side on a side of the outer electron emitting devices, the side extending generally along the first direction, wherein the first conductor is electrically connected to the first wire.
The invention also provides an image forming apparatus comprising: the electron source described above; and a fluorescent member for emitting light upon application of electrons emitted from the electron source.
The invention also provides a method of manufacturing an electron source having a plurality of wires and a plurality of electron emitting devices connected to the wires, comprising the step of: forming a wiring pattern, and a conductor pattern similar to the wiring pattern in an area different from an area where the wiring pattern is formed, by a screen printing method.